1. Technical Field
The embodiments herein generally relate to mobile television (TV) technologies, and, more particularly, to pilot-aided orthogonal-frequency-division-multiplexing (OFDM) systems.
2. Description of the Related Art
Frequency division multiplexed communications systems transmit upwards of thousands of carrier signals simultaneously to communicate information. In the case of OFDM systems the transmitted carrier signals are orthogonal to each other to avoid or minimize mutual interference. In an OFDM system, each transmitted carrier signal may be used to transmit a different unit of data (e.g., symbol) in parallel. In pilot-aided OFDM systems, carriers of the OFDM symbol are modulated, at the transmitter, with data symbols in addition to pilot symbols known to both the transmitter and the receiver. At the receiver, these pilots are used to estimate the channel at the corresponding carrier positions. The pilot positions are not necessarily fixed from one OFDM symbol to another. If the pilot position is fixed across the OFDM symbols, it is called a “continuous” pilot; if it is variable, it is called a “scattered” pilot. FIG. 1 shows an example of an OFDM symbol structure where there are both continuous and scattered pilots. In order to recover the entire channel (all the carriers) from the estimated carriers at the pilot positions, the estimated carriers are interpolated to obtain the entire channel.
For each transmitted carrier signal, an OFDM receiver attempts to compensate for the distortion induced by the transmission channel. Typically, this involves a channel estimation operation and a channel compensation operation. In order to assist a receiver in overcoming multipath distortion, pilot signals having known data patterns are transmitted. Moreover, the pilot signals are used to support channel estimation operations, which generally attempt to estimate the amplitude and phase distortion introduced by the communications channel. The pattern structure of the pilots can generally be in any pattern provided that the Nyquist sampling criteria for the communication channel's impulse response and rate of change are satisfied. Furthermore, the number of pilots transmitted is often a function of the expected multipath distortion delay and the anticipated rate of change in channel conditions. However, it is desirable to minimize the number of pilots transmitted since the transmission of a pilot precludes the transmission of data in the transmission slot used to transmit the pilot.
The pilots represented in FIG. 1 are distributed in the frequency domain and the time domain. As shown in FIG. 1, there are symbol transmission periods during which no pilots are transmitted or received. Furthermore, the received known pilot signals are used to estimate the channel distortion at the time and frequency of each pilot. For each OFDM transmitted carrier signal a channel estimate is typically required for channel compensation purposes. Thus, where no pilots are located, a channel estimate has to be generated. Pilots for these frequency/time slots may be generated using interpolation on the received pilots in the time and/or frequency domain. Moreover, the pilot interpolation can be performed using a number of known techniques. For example, one can perform a simple linear interpolation between pilot data points or more sophisticated cubic interpolation. Alternatively, one may simply fill in the gaps between pilot bins by performing a low pass filter (LPF) operation on the received pilot data points. In this context, a “bin” represents data or a set of data corresponding to an individual carrier frequency.
Unfortunately even with pilots spaced sufficiently close enough in the frequency and time domains to meet the channel's Nyquist criterion, filling in the channel estimate between the known pilot bins may still be prone to error due to additive noise. Conventional channel estimation techniques attempt to solve this problem in the following manner. The OFDM symbols are received, the pilots are extracted, then averaged over many OFDM symbols. Then, once sufficient averaging in performed providing a reduction in noise corruption, the channel is interpolated between the pilots. After this, the received signal on a given channel is multiplied by the inverse of the corresponding channel estimate in an attempt to remove multipath and/or other distortion introduced by the communications channel. This technique works well except that it depends on a relatively long integration time to reduce the noise corruption. Accordingly, this delay is undesirable since it increases the time between when a carrier recovery signal lock is first achieved and when received symbols may be decoded in a reliable manner. Therefore, there remains a need for a new interpolation technique for OFDM scattered pilots.